Disclosed in DE-AS No. 1 277 159 is an apparatus of the type defined in the introduction, in which the basin is a channel-type structure. Basins of this type are usually concrete structures.
For aerating the active sludge contained in the basin, the apparatus is provided with grid-shaped bottom aerators suspended from a bridge structure extending transversely of the channel-type basin. In order to ensure adequate aeration of the active sludge over the full length of the basin, the bridge structure is mounted for reciprocating movement in the longitudinal direction of the channel. It is also suggested to attach the bottom aerators to a float which may be dragged through the basin without being supported by the bridge structure carried by the basin walls.
Basins of this type are constructed as so-called compact installations, but sometimes also as large scale installations. It has become evident, however, that a sufficiently safe control of the active sludge required for the sewage purification can be achieved in an economical manner only, if at all, in the case of the smaller compact installations. In the case of larger installations there is always the danger that the active sludge in certain areas dies off in an uncontrolled manner, leading to unstable conditions in the basin during the purification of sewage.
The smaller installations as well as the larger installations suffer from the disadvantage that their construction and operation are rather expensive. The treatment basins, which in most cases are concrete structures, are expensive to build, and the mechanical installations for moving the bottom aerators and controlling the aeration process are complicated and subject to failure. In addition to high capital investment this results in high operating costs mainly due to high manpower costs.
The high investment and operating costs of an installation of this type frequently lead to several communities forming a joint venture. This, however, necessitates the construction of sewers from the participating communities to the common sewage treatment plant. The costs of such sewers often exceed those of the treatment plant itself.
In view of these disadvantages, large-scale earth basin installations with and without artificial aeration have been developed for plants serving up to about 10,000 PUV (population unit value). Pure earth basins without artificial aeration have conventionally been built only for a few hundred PUV. In these basins there prevail undefined decomposition conditions between aerobic and anaerobic areas, so that an active sludge in the usual sense cannot develop. Since the absorption of oxygen takes place only at the surface, installations of this type have to have a very large surface area, and their operation in winter is unsatisfactory. The dwelling time of the sewage in these installations is usually 30 days and above.
The installations developed from such earth basin installations and provided with line aerators or other devices, such as spot aerators, permitted to reduce the surface area requirements, they are still suffering, however, from ill defined conditions as concerns their operation, resulting in dwelling times of domestic sewage of 10 to 20 days in the aerated sector, and of up to 30 days in the installation as a whole. In installations operating under higher load one has to accept extremely long dwelling times again requiring excessively large installations. While aerobic conditions prevail in certain areas of these installations, they are not controllable, and areas with anaerobic conditions cannot be avoided.